Method of securing end shields to the stator assembly of a dynamoelectric machine

ABSTRACT

A system and method of securing the end shields of a dynamoelectric machine (e.g., a fractional horsepower electric motor) to its stator assembly with the rotor assembly of the motor held centered within the bore of the stator assembly and journalled in the motor&#39;s end shields. The system involves the use of threaded fasteners (preferably self-tapping screws or the like) which are inserted through enlarged holes provided in the end shield into mating holes provided in the end face of the core of the stator assembly. With the rotor accurately centered in the stator bore, the fasteners are tightened thereby to firmly and securely draw the end shield into firm gripping engagement with the end face of the stator core while the desired uniform air gap between the rotor and the bore of the stator assembly is maintained.

This is a division of application Ser. No. 956,355, filed Oct. 31, 1978,now U.S. Pat. No. 4,306,168.

BACKGROUND OF THE INVENTION

This invention relates to a system and a method of securing the endshields of a dynamoelectric machine (e.g., fractional horsepowerelectric motor) to the stator assembly of the motor with the rotor shaftjournalled in bearings carried by the end shields and with the rotorproperly positioned (i.e., centered) within the bore of the statorassembly so as to provide a uniform air gap therebetween.

Typically, a fractional horsepower motor includes a stator assemblyhaving a stack of laminations (preferably made of sheets of suitablesteel or other ferro-magnetic material) punched to have a central boreand a plurality of winding receiving slots extending generally radiallyoutwardly from the bore. The laminations are stacked to form a core andare securely fastened together. Welding or epoxy adhesive work well asfastening methods, for example. A rotor assembly including a rotor,typically of a squirrel cage design, and an axial shaft extending fromboth ends thereof is provided. The rotor is intended to be receivedwithin the bore of the stator core so as to be centered therewithin andto have a substantially uniform air gap between the rotor and portionsof the stator core forming the bore. These motors typically include apair of end shields adapted to be secured to the ends of the statorassembly. The end shields each include a bearing (e.g., a suitableanti-friction bearing) in which one end of the rotor shaft is journalledso that the bearings rotatably support the motor within the statorassembly.

Heretofore, a variety of means and procedures have been used to locate(i.e., center) the rotor within the bore of the stator assembly and tosecure the rotor in this centered position. Typically, duringmanufacture of such an electric motor, a rotor is inserted into the boreof its respective stator assembly and temporary shims are fitted so asto accurately center the motor with respect to the bore. The bearingsand the end shield are then fitted on the rotor shaft and the endshields are fitted on the stator assembly. With the end shields and therotor in proper position, and end shields are then secured to the statorand the temporary shims are removed.

Certain of the various means heretofore used to secure the end shieldsto the stator assembly will be now discussed. In many motors, so-calledthrough-bolts extend through openings in the stator core and throughopenings in both of the end shields. Typically, the shanks of thethrough-bolts are somewhat smaller than the diameter of the openings inthe stator core and the openings in the end shield so as to permit theend shields to be shifted relative to the stator core thereby to enablethe rotor to be properly centered within the bore of the stator assemblyand so as to provide the desired uniform air gap therearound. Throughbolts are then tightened to a predetermined torque level while the rotoris shimmed in its desired centered location. The degree to which thethrough-bolts are tightened (i.e., a predetermined torque level)preloads the bolts so as to apply a clamping force between the endshields and the stator assembly. As the end shields are drawn into firmengagement with the ends of the stator assembly the friction between theend shields and the stator assembly holds the end shields in place.

Certain problems, however, have been noted with this above-noted priorart system of securing the end shields in position on the statorassembly. First, the cost of the through bolt fasteners is relativelyhigh. Second, only the friction of the end shields on the statorassembly holds the rotor centered within the bore of the stator. If therotor shaft or end shield shifts either during shipping or during use ofthe motor, it is possible for the rotor to move from its centeredposition thus upsetting the uniform air gap between the rotor and boreof the stator. In some instances, if the air gap becomes too small or ifthe rotor physically touches or "strikes" the stator, the motor fails.Since the end shields of these prior art motors are dependent entirelyon the frictional force between the end shields and the stator core tohold them firmly in place, the impact force which the end shields willwithstand and yet hold the rotor centered is dependent on the preload ofthe through-bolts. Prior motors utilizing through-bolt construction haveexperienced problems with the preload of the through-bolts relaxing overtime. This is due in part to the fact that the stator assembly istypically dipped in an insulating varnish solution and dried. Thevarnish, of course, coats the outer end faces of the stator assembly andis present between the laminations of the stator assembly. When thethrough bolts are drawn tight, the varnish between the end shields andthe stator core and the varnish between the laminations tends to seepout under compressive load, but still may be present after varnish set.In use, motor operating temperatures can soften the varnish so that itagain becomes flowable and "creeps" under the compressive load. Thiscreeping of the varnish causes the preload in the through-bolts torelax, thus lessening the frictional force available to hold the endshield in place on the stator core.

A second method or system of securing the end shields to the statorassembly involves bonding the end shields to the stator assembly oncethe rotor and end shields have been properly positioned relative to thestator and once the rotor is centered relative to the stator bore.Typically, a suitable epoxy adhesive or the like is used to bond the endshields in place to the stator assembly. While this method of bondingthe end shields in place may offer some advantages in manufacture,improperly bonded end shields may easily be separated from the statorassembly if inadvertently struck during shipping or during use of themotor. Also, there is no ready or practical way to field repair or toeconomically rebuild the motor with bonded in place end shields.

Reference may be made to such U.S. Pat. as U.S. Pat. Nos. 2,423,750,3,176,172, 3,320,660, 3,437,853, 3,707,037, and 3,966,278 which disclosevarious methods and systems of securing end shields in place to thestator assembly of electric motors or the like.

SUMMARY OF THE INVENTION

Among the several objects and features of this invention may be notedthe provision of a system for and a method of securing the end shieldsof a dynamoelectric machine (e.g., a fractional horsepower electricmotor) to the stator assembly of the motor in which the end shields aresecurely attached and in which the rotor assembly is maintained in acentered position relative to the bore of the stator assembly even whensubjected to extreme impact loads;

The provision of such a system and method which is less expensive thancertain other known securement systems and which speeds up production ofmotors thus reducing the cost of the motors;

The provisions of such a system and method which may be readilyincorporated into the manufacture of a wide variety of electric motordesigns;

The provision of such a system and method which results in a motor whichmay be readily disassembled for repair and which may be readilyreassembled in the field without damage to the parts of the motor;

The provision of such a system in which the fasteners provide amechanical interlock (as distinguished from a mere frictionalsecurement) between the end shield and the stator assembly once thefastening system has been properly installed and tightened;

The provision of such a system which is appreciably stronger than otherfastening systems against resisting impact loads applied to the endshield or the rotor shaft;

The provision of such a system and method which enables uniform air gapsto readily be established between the rotor and the bore of the statorassembly prior to final securement of the end shield in place and whichreliably maintains the desired air gap throughout the service life ofthe motor;

The provision of such a system and method in which the force securingthe end shields does not appreciably relax during the service life ofthe motor;

The provision of such a system and method which increases productivityof the manufacture of electric motors; and

The provision of such a system and method which allows one end shield tobe installed or removed from the stator assembly independently of theother.

Briefly, a dynamoelectric machine (e.g., a fractional horsepowerelectric motor) typically has a stator assembly including a stack oflaminations of suitable magnetic material forming a core. Each of theselaminations has a central opening therethrough so that the core has acentral bore. The motor further includes a rotor assembly having a rotoradapted to be received within the bore of the stator assembly and arotor shaft extending endwise from the rotor. Still further, the typicalmotor includes at least one end shield having a bearing for receivingand journalling the rotor shaft, the end shield being adapted to besecured in fixed relation relative to the stator assembly. Wherein, theimprovement of this invention comprises a system including a pluralityof holes in the core of the motor spaced around the bore and facinggenerally outwardly of the core at at least one end thereof. A pluralityof openings in the end shield is so arranged so as to generally matewith the holes in the stator assembly when the end shield is fitted ontothe stator assembly in its assembled position. A plurality of fastenersis provided, one for each of the end shield openings, the fastenersbeing threadably engageable with the core when inserted into theopenings in the end shield and into a corresponding hole in the core anddrawing the end shield into firm engagement with the stator assemblywhen the fasteners are tightened.

The method of this invention of assembling a dynamoelectric machine,such as a fractional horsepower motor or the like will now be generallydescribed. Typically, the motor has a stator assembly including a coremade up of a stack of laminations of suitable magnetic material, thecore having a central bore extending therethrough and a plurality ofholes spaced around the bore and facing generally outwardly of the core.A rotor assembly including a rotor is adapted to be rotatably receivedin the bore and a rotor shaft extends endwise from the rotor. At leastone end shield is provided, the latter having a bearing adapted toreceive and journal the rotor shaft. The method of this inventioncomprises the steps of inserting the rotor assembly into the statorassembly with the rotor being received in the bore. The rotor iscentered with respect to the bore so that the rotor has an air gapbetween the rotor and the core of substantially uniform thicknesstherearound. An end shield is then installed on the stator assembly andon the rotor assembly so that the rotor shaft is journalled in thebearing of the end shield and so that the end shield bears against thecore. The core has a plurality of holes therein which are substantiallyaligned with holes in the portions of the end shield which engages thecore. Fasteners are inserted into the end shield holes and into theholes in the core and tightened thereby to draw the end shield into firmgripping engagement with the core so as to securely hold the end shieldin place with respect to the core and to journal the rotor shaftrelative to the core such that the uniform air gap is maintained.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a dynamoelectric machine (e.g., afractional horsepower electric motor) having a stator assembly, a rotorassembly and a pair of end shields secured to opposite ends of thestator assembly in accordance with the system and method of thisinvention for journalling the rotor assembly for rotation about itscentral longitudinal axis within the stator assembly, a portion of themotor being broken away to show the rotor body, the bore of the stator,and the air gap therebetween;

FIG. 2 is an enlarged cross-sectional view of a portion of the motorshown in FIG. 1 illustrating the system of this invention for securingthe end shields of the motor to the stator assembly;

FIG. 3 is a view similar to FIG. 2 illustrating a prior art system ofsecuring the end shields to the stator assembly in which a so-calledthrough-bolt is inserted through openings in both of the end shields andthrough an opening in the stator assembly so as to clamp the end shieldsin place on the stator assembly;

FIG. 4 is a greatly enlarged view of the system of this inventionillustrating a self-tapping fastener threadably inserted into an openingin the core of the stator assembly of the motor and firmly engaging theend shield of the motor;

FIGS. 5A-5C illustrate the steps of the method of this invention forsecuring the end shields of a motor in place on the stator assembly; and

FIG. 6 is a graph comparing the impact force applied to the end shieldversus the average change in the air gap of the motor for both motorsassembled in accordance with the present invention and motors assembledin accordance with the systems of the prior invention, such asillustrated in FIG. 3.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings, a dynamoelectric machine (e.g., afractional horsepower electric motor) is indicated in its entirety atreference character 1. The motor is shown to include a stator assembly,as generally indicated at 3, a rotor assembly 5, and a pair of endshields, as generally indicated at 7a, 7b, secured to the statorassembly for journalling the rotor assembly within the stator assembly.Motor 1 may be any conventional electric motor or other dynamoelectricmachine. The stator assembly is composed of a core 9 made up of aplurality of laminations 11 (see FIG. 4) arranged in a stack. Each ofthe laminations is preferably a pre-punched plate-like member having acentral opening (not shown), a plurality of slots extending radiallyoutwardly from the central opening, and a plurality (four are providedin the laminations as shown) of holes 13 therethrough arranged aroundthe central opening. The laminations are made of a suitable magneticmaterial, preferably of sheet steel or other suitable ferro-magneticmaterial. With the laminations stacked so as to form core 9, the centralopenings are coaxial so as to form an axial circular bore 15 (seeFIG. 1) extending through the stator assembly and the radial slots formopenings for receiving coils of wire which constitute the windings W(see FIG. 5A) of the motor, and holes 13 are in alignment with oneanother so as to form holes extending through the stator core. As shown,holes 13 are generally parallel to the axis of the bore 15. It will beunderstood, however, that holes 13 need not extend through the core. Thelaminations are typically secured to one another, as by welding, so asto form a unitary core 9.

Rotor assembly 5 includes a rotor body 17, which may be a conventionalsquirrel cage type rotor body, having a rotor shaft 19 concentrictherewith and extending outwardly from each end of the rotor body. Thelatter is somewhat smaller than the inside diameter of bore 15 in core9. Thus, when rotor body 17 is centered within bore 15, a uniform airgap G (see FIG. 1) will result between the rotor body and the core.

End shields 7a, 7b are substantially identical and are preferablyunitary die castings of, for example, a suitable aluminum alloy or thelike. Other end shield constructions are compatible with the broaderaspects of this invention. Each end shield has a central portion orpillow 21 for receiving an anti-friction bearing 23. The race of thebearing is so sized as to receive a corresponding portion of rotor shaft19. Each end shield is further shown to comprise a plurality of legs 25extending from the central pillow 21 thereof toward stator assembly 3.The outer ends of legs 25 have feet 27 formed thereon which engage theend face of the stator assembly. As best shown in FIG. 2, feet 27 fit onthe end face of core 9 and each of the end shield feet has acorresponding opening 29 therethrough adapted to be in register with(i.e., to be generally in line with) a corresponding hole 13 in core 9when the end shield is properly fitted on stator assembly 3 in itsassembled position.

As used in this disclosure, the term "end shield" refers, in a genericsense, to denote any type of structure which is fitted onto the end ofthe stator assembly of a motor so as to rotatably journal the rotorassembly in the stator. This term would apply to any structure whetheror not it has a shielding function.

In accordance with the system of this invention, with rotor assembly 5fitted within bore 15 of core 9, shims S (see FIGS. 5A-5C) are fittedbetween the rotor and the bore so as to form a uniform air gap Gtherearound provided. These shims are adapted to be inserted into thegap between rotor body 17 and core 9 so as to hold the rotor centeredwithin bore 15. End shields 7a, 7b are then installed on shaft 19 sothat the rotor shaft is received within the inner races of bearings 23and the end shields are fitted on rotor assembly 3 so that their feet 27engage core 9 (preferably the end face thereof) with holes 29substantially in register with holes 13 in core 9. Fasteners F,preferably self-tapping fasteners, are then inserted into each of theopenings 29 and into each of the corresponding holes 13 and tightened.As shown in FIG. 4, each fastener F is shown to have a tapered shank 31with self-tapping threads 33 thereon for progressive threadableengagement with laminations 11 of core 9 adjacent hole 13 as thefastener is threaded into the hole. Each of the fasteners has a head 35with an outwardly projecting flange 37, the latter being considerablylarger than opening 29 in end shield foot 27 so as to engage the endshield when the fastener is tightened. Head 35 has a slot 39 therein forreception of a suitable fastener driving tool (e.g., a screw driver orthe like) for tightening of the fastener. Other head 35 designs may beemployed, if desired. Still further, fastener flange 37 has an annularprojection delimiting a tooth 41 on its bottom face which dig into theend shield foot 27 adjacent openin 29 as the fastener is drawn tight soas to at least partially mechanically interlock the fastener to the endshield and to greatly inhibit sliding movement of the end shieldrelative to the fastener.

As shown in FIG. 4, opening 29 is somewhat larger than shank 31 offastener F so as to permit the end shields to be shifted on core 9 withfasteners F in place (but not tightened) and to accommodate a range ofalignments between the rotor and stator assemblies. As each fastener Fis drawn tight, leg 27 of end shield 7a or 7b is drawn into firmgripping engagement with the outer end face of core 9. As mentionedabove, tooth 41 digs into the end shield and provide at least somemechanical interlocking between the end shield and the core. Furtherinterlocking is provided due to the tightening of the fastener causing,in some instances, a slight deformation (e.g., an outward protrusion),as indicated at 42 in FIG. 4, of the outside laminations 11 of core 9adjacent hole 13. As shown in FIG. 4, these protrusions 42 of the outerlaminations are greatly exaggerated for purposes of illustration, but itwill be seen that the outer laminations are nevertheless drawn outwardlyand are drawn into opening 27 of end shield leg 29. These protrusionstherefore hold the end shield in its desired position relative to thestator assembly and effectively prevent sideways movement of the endshield relative to core 9 in the event it is impacted in sidewisedirection or in the event a load, such as a pulley load, is applied tothe motor. It will be noted that if the end shield is forced sideways inany direction relative to the stator core, these protrusions of theoutside laminations of the core force the end shield to move outwardlyaway from the end face of the core and this in turn places additionaltension loading on fasteners F. This additional tension loading on thefasteners, of course allows the fasteners to more securely hold the endshield in fixed position fixed on the core even when the fasteners aretorqued to relatively low preload levels. p A typical prior artthrough-bolt end shield attachment system is illustrated in FIG. 3. Core9' of motor 1' has a plurality of holes 13' therethrough. End shields7a', 7b' of motor 1' have feet 27' which bear against the end faces ofcore 9'. A so-called through bolt B is inserted through holes 29' in theend shield feet and through holes 13' in core 9'. It will be noted thatholes 13' and holes 29' are, of necessity, somewhat larger than theoutside diameter of the shank of through bolt B. With the end shield7a', 7b' properly positioned on core 9' with the rotor centered withinthe bore of the stator assembly, through-bolts B of the prior art endshield securement system are tightened to a predetermined torque levelso as to preload the bolts to a desired level and so as to draw the endshield feet into firm engagement with the end faces of core 9. However,because the through-bolts are somewhat smaller than the holes throughcore 9' and through end shield feet 27', the end shields are dependententirely on the tension (i.e., the preload) of through-bolts B togenerate sufficient frictional force between the end shield and the coreso as to firmly hold the end shields in place.

As heretofore mentioned, it is conventional in the manufacture ofelectric motors to insulate the stator assembly by dipping it into abath of insulating varnish and by baking the varnish so as to provide aninsulation coating on the stator assembly. This varnish, of course,coats the end faces of the core and is disposed between the laminationsof the core. Two problems have been noted upon tightening theabove-discussed prior art through-bolt fastening system. First, thevarnish under the heads and nuts of the through-bolts and the varnishbetween the laminations of the core tends to creep under compressionloading thus causing the preload in the through bolts to relax. Uponrelaxation of the preload in the through bolts, the frictional force inholding the end shields in place on the core is appreciably lowerthereby allowing the end shields to be more readily displaced or movedrelative to the core. This makes movement of the end shields relative tothe core more likely upon the end shield or the rotor shaft beingimpacted either during shipping or during the service life of the motor.Movement of the end shield, of course, causes shifting of the rotorwithin the bore which may cause possible "rotor strike" malfunctions ofthe motor.

In sharp contrast, the fastening system of the present inventionovercomes the above-noted problem of prior art fastening systems. Inparticular, it will be noted that the threaded shank S1 of each fastenerF of the present invention is firmly embedded in a respective hole 13 ofcore 11. Thus, fastener F has no play or movement relative to core 9 andthe portion of fastener shank 31 which extends out beyond the end faceof the core constitutes a stiff cantilever member for holding the endshield firmly in place. With fastener F tightened, and with teeth 41 ofthe fastener dug into the metal of the end shield feet 27 adjacent holes29, the end shield and the core are mechanically interlocked not only byteeth 41, but by the outwardly protruding portions 42 of thelaminations. Still further, the stiff cantilevered shank of the fastenerfurther aids in holding the end shield in position.

Further in accordance with the system of this invention, rotor 17 iscentered within bore 15 prior to securement in place of the end shieldsby fasteners F by means of shims S (see FIGS. 5A-5C) which are adaptedto be inserted into gap G between the rotor and the bore. As shown,shims S are elongate rod-like shims having a thickness conforming to thenominal thickness of the desired air gap G. The shims may be insertedinto the air gap at spaced intervals around the rotor by inserting themthrough openings (e.g., the openings formed by legs 25) in end shields7a, 7b. For example, the shims may be inserted or removed after the endshields have been fitted in place or the shims may be inserted prior tofitting of the end shields in place.

The method of this invention for assembling an electric motor includesthe steps of first inserting a rotor assembly 5 into the bore 15 of astator assembly 3 (as shown in FIG. 5A). Then, end shields 7a, 7b arefitted on the stator assembly so that the ends of the rotor shaft 19 arereceived within the inner races of bearings 23 and so that feet 27 ofthe end shields fit on their respective end faces of core 9 withopenings 29 in the end shield in register with holes 13 in the core.Shims S are then inserted through openings in the end shields into airgap G between rotor 17 and bore 15 thereby to center the rotor withinthe bore and to establish a desired air gap G of substantially uniformthickness around the rotor. It will be understood that the shims couldbe positioned between the rotor and the bore prior to fitting the endshields in place. Next, fasteners F are inserted into their respectiveopenings 29 in end shields 7a, 7b and into mating holes 13 in core 9 andare driven (i.e., tightened) so that the fasteners self-tap into thecore laminations 11 and so as to firmly draw the end shields intogripping engagement with core 9 while the rotor is held in its desiredposition with respect to bore 15 by shims S. After securement of the endshields in place, shims S are removed (i.e., pulled) from the rotor andthe stator assembly.

EXAMPLE

A test was conducted to compare the impact strength of motors assembledin accordance with this invention and of motors assembled in accordancewith the prior end shield fastening systems with the rotor centering ofthe motors. 106 dishwasher motors were assembled utilizing the system inthe method of this invention as heretofore described. Fasteners F ofdifferent motors were tightened to predetermined torque levelscorresponding to various preloads of fastener F as follows:

    ______________________________________                                                     ACTUAL TORQUE                                                    GROUP        RANGE          QUANTITY                                          ______________________________________                                        Low torque   50-60 in./lb.  40                                                Medium torque                                                                              60-75 in./lb.  20                                                Medium torque of                                                                           60-75 in./lb.  20                                                reassembled motor                                                             High torque  95-115 in./lb. 26                                                ______________________________________                                    

The above motors were tested against forty conventional dishwashermotors having their end shields secured in place on their statorassemblies by through-bolts substantially as shown in FIG. 3. Thesethrough-bolts were tightened to have a preload torque ranging between145-135 in./lb.. The motors constructed in accordance with the systemand method of this invention and the motors constructed in accordancewith the prior art were subjected to impact loads applied to their endshields or rotor shaft and the shift in the air gap between the rotorand the core of the motors was recorded. In FIG. 6, the average shift inair gap versus the impact force applied to both the prior art motors andthe motors made in accordance with the present invention are shown.Suprisingly, the average shift in air gap of motors constructed inaccordance with the present invention is appreciably lower than theprior art through bolt securement system at all impact levels testedeven though fasteners F were tightened to lower preload levels (e.g.,torque levels). Additionally, it will be noted that the prior artfastening system resulted in a shift of the air gap of such magnitude asto cause rotor strike upon start up of the motor when the motor had beensubjected to impact loads in excess of 15 ft./lb.. When the prior artmotors were subjected to impact loads in excess of 18 ft./lb., staticmotor strike could occur. It is significant that the average shift inair gap at all impact levels tested of motors made in accordance withthe system and method of the present invention was well below the airgap shifts required to cause either static or start up rotor strike.Also, the system and method of this invention for securement of endshields in place has a significantly higher resistance to impact loadswhich may be incurred during shipping or during the service life of themotor. Thus, the motors assembled in accordance with the method andsystem of this invention are not as sensitive to initial tighteningtorque or to preload of the fasteners as are motors utilizing the priorart through-bolt fastening system.

Still further, it is pointed out that fasteners F of the present systemand method threadably engage the outermost laminations 11 of core 9 andthus substantially eliminate any effect that creep between laminations11 of the core may have. Thus, the preload in fasteners F of the presentinvention are not as likely to relax as are prior through-bolt fastenersystems due to creep of the varnish material from between the corelaminations.

While the system and method the present invention has been hereindescribed as preferably using self-tapping fasteners F to secure the endshields to the core, it will be understood that conventional threadedfasteners inserted into threaded holes in the core may be used.

It will be noted that end shields 7a, 7b may be installed or removedfrom stator assembly 3 independently of one another.

In view of the above, it will be seen that the several objects of theinvention are achieved and that other advantageous results are attained.

As various changes could be made in the above constructions and methodswithout departing from the scope of this invention, it is intended thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

We claim:
 1. A method of assembling a dynamoelectric machine, such as afractional horsepower motor or the like, said motor having a statorassembly including a core made up of a stack of laminations of suitablemagnetic material, said core having a central bore extendingtherethrough, a rotor assembly including a rotor body adapted to berotatably received in said bore and a rotor shaft extending endwise fromsaid rotor body, and at least one end shield, the latter having abearing adapted to receive and to journal said rotor shaft, said methodcomprising the steps of:inserting said rotor assembly into said statorassembly with said rotor being received in said bore; centering saidrotor body within said bore so that said motor has an air gap betweensaid rotor body and said core of substantially uniform thicknesstherearound; installing said end shield on said stator assembly and onsaid rotor assembly so that said rotor shaft is journalled in saidbearing and so that said portions of said end shield bear against anadjacent portion of said core, said end shield portions having holestherethrough and said adjacent portion of said core having holes thereinin substantial alignment with said holes in said end shield portionswhen said end shield is fitted on said stator assembly; and insertingthreaded fasteners into said end shield holes and into said holes insaid core and tightening said fasteners so as to draw said end shieldinto firm gripping engagement with said core and so as to bend outwardlyat least the outermost lamination adjacent said holes in said core withthe outwardly bent portion protruding at least partially into a matingopening in said end shield thereby to securely hold said end shield inplace with respect to said core and to journal said rotor shaft relativeto said core so that said uniform air gap is maintained.
 2. The methodof claim 1 wherein said step of centering said rotor is accomplished byinstalling shims between said rotor and the surface of said coredefining said bore, and wherein said method further comprises removingsaid shims after said fasteners have been tightened.
 3. The method ofclaim 1 further comprising tightening said fasteners to a predeterminedtorque level thereby to ensure said end shield is firmly held in placeon said stator assembly.